Mixers are used in communication devices to down step or up step the frequency of their input signal. In receivers, mixers are utilized to mix a received signal with a locally generated reference or injection signal to produce a difference signal. This difference signal is further processed and demodulated according to a variety of demodulation schemes before being presented to a user. Mixers in general, and balanced mixers in particular, are well known in the art and have been used by radio designers extensively. A variety of balanced mixers are available. Some with one balanced port, others with two or three balanced ports. Doubly balanced mixers have two of their ports balanced. Triply balanced mixer have all three of their ports balanced. The advantages of balanced mixers include improved spurious response, improved noise degradation, simplicity of their matching networks, and so forth. Various topologies have been used in realizing mixer requirements. In general, however, a doubly balanced mixer includes a balanced or differential radio frequency (RF)input port and a balanced or differential local oscillator (LO)input port where each port accommodates a balanced signal to be mixed with the other. Integrated doubly balanced mixers are generally based on a structure known as Gilbert Cell. The Gilbert structure has limitations to its intermodulation (IM) distortion performance, noise figure, current drain performance, and a trade off amongst these three criteria. These limitations have limited the use of balanced mixers in communication devices with stringent performance requirements.
Referring to FIG. 1, a circuit diagram of a mixer 100 is shown utilizing presently known circuit topologies. The mixer 100 includes transistors 106, 108, 110, and 112 that are ultimately coupled to balanced local oscillator inputs LO(+) and LO(-). Coupled to these transistors are two input gates, transistors 114 and 116 which couple balanced radio frequency inputs RF(+) and RF(-). A balanced RF signal applied to the base of transistors 114 and 116 is switched at the rate of the local oscillator signals applied to transistors 106, 108, 110 and 112. The switching of the RF signal results in a signal appearing at the output Out (+) and Out (-) equal to the difference of the RF and LO signals. Two degeneration resistors 118 and 120 couple the emitters of transistors 114 and 116 to a current source shown by a transistor 122 and a resistor 124. The degeneration resistors 118 and 120 are used to provide improvements in the third order IM at the expense of some gain and noise figure. It can be Shown that the third order IM intercept point is given by the following equation: ##EQU1## Where V.sub.T =kT/q
K=1.38.times.10.sup.-24 (Boltzman's Constant) PA1 T=Kelvin Temperature PA1 q=1.6.times.10.sup.-19 (Electron Charge) PA1 Ri=RF port input impedance.
Since the gain of the mixer 100 is set essentially by the value of R.sub.E it can be concluded from the equation that with this circuit configuration of a given gain the only way to improve intercept point is to increase the current drain or to reduce Ri. Normally, input impedance is set by other criteria and cannot be changed or if it is changed will result in a no win shift in all performance parameter values. With high demands on performance improvements and reduced current consumption it can be seen that IM and noise figure can be limitedly improved at the cost of increased current drain. It is therefore desired to have a mixer circuit that has improved IM and noise figure performance without a significant increase in current drain.